Since the advent of charge coupled devices, a new generation of signal processing techniques has evolved. Progress has shown that the silicon charge transfer device is one of the most important concepts in realizing visible imagers. Extending this technology to the infrared field has recently been the subject of intensive study in focal plane design for advanced military systems. Two approaches can essentially accomplish the same goal, namely, hybrid and monolithic focal planes. Hybrid focal planes, which involve mating sensitive intrinsic detectors to Si signal multiplexers or equivalent, are a nearterm solution to system problems. In monolithic focal planes, photon detection and signal processing are accomplished in one semi-conductor and consequently, are cost effective in the long run.
The building blocks of monolithic infrared focal planes need not be restricted to extrinsic Si; other semiconductors can be developed and tailored to specific needs of systems. In fact, besides potentially higher temperature of operation, infrared focal planes using intrinsic semiconductors have an additional important advantage over Si in that a heterojunction structure can be designed to detect low infrared (IR) energy in the narrow gap semiconductor and transfer the resulting charges to the wider gap semiconductor for signal processing.
Mercury-cadmium telluride, (Hg.sub.1-x Cd.sub.x Te), which is an important intrinsic semiconductor for the fabrication of photovoltaic and photoconductive infrared detectors, can be tailored to cover a wide spectral range from visible to over 30 .mu.m. Although numerous techniques for growing HgCdTe bulk crystals have successfully been developed, the various techniques for growth of HgCdTe epitaxial layers have had limited success until modified liquid phase epitaxial (LPE) techniques were developed. LPE solves two typical problems encountered in the HgCdTe bulk crystal growth: compositional non-uniformity, and long annealing time to reach homogeneity. HgCdTe epitaxial layers also have the advantage of being suitable for making backside-illuminated detectors. In addition, LPE techniques can be adopted for growing HgCdTe multilayers which can be used for the fabrication of monolithic HgCdTe detectors and charge-coupled devices (CCD).
Advantageous would be a process for growing a layer of CdTe on a HgCdTe epitaxial layer that has been grown on a substrate of CdTe. The structure resulting from the above process offers the advantages of a backside-illuminated system since the CdTe substrate is transparent to IR. Since CdTe has a wide bandgap (Eg=1.6 eV), the dark current of a charge-coupled device would be low. The CdTe layer could serve as a signal processer after a low level of IR is detected by a HgCdTe layer.
Therefore, an object of this invention is to provide a method of growing by liquid-phase epitaxial growth multilayers of CdTe/Hg.sub.1-x Cd.sub.x Te with an x value from about 0.17 to about 1.0 on a CdTe substrate so that some mercury is always present in said Hg.sub.1-x Cd.sub.x Te epitaxial layer.
Another object of this invention is to provide a process for growing multilayers of CdTe/Hg.sub.1-x Cd.sub.x Te on a CdTe substrate which yields a structure, a CdTe/HgCdTe heterojunction, sensitive to .about.2.8 micrometers at 77.degree. K.